185 research outputs found
Simulating binary neutron stars: dynamics and gravitational waves
We model two mergers of orbiting binary neutron stars, the first forming a
black hole and the second a differentially rotating neutron star. We extract
gravitational waveforms in the wave zone. Comparisons to a post-Newtonian
analysis allow us to compute the orbital kinematics, including trajectories and
orbital eccentricities. We verify our code by evolving single stars and
extracting radial perturbative modes, which compare very well to results from
perturbation theory. The Einstein equations are solved in a first order
reduction of the generalized harmonic formulation, and the fluid equations are
solved using a modified convex essentially non-oscillatory method. All
calculations are done in three spatial dimensions without symmetry assumptions.
We use the \had computational infrastructure for distributed adaptive mesh
refinement.Comment: 14 pages, 16 figures. Added one figure from previous version;
corrected typo
Boundary conditions for hyperbolic formulations of the Einstein equations
In regards to the initial-boundary value problem of the Einstein equations,
we argue that the projection of the Einstein equations along the normal to the
boundary yields necessary and appropriate boundary conditions for a wide class
of equivalent formulations. We explicitly show that this is so for the
Einstein-Christoffel formulation of the Einstein equations in the case of
spherical symmetry.Comment: 15 pages; text added and typesetting errors corrected; to appear in
Classical and Quantum Gravit
Relativistic Hydrodynamics for Heavy--Ion Collisions -- II. Compression of Nuclear Matter and the Phase Transition to the Quark--Gluon Plasma
We investigate the compression of nuclear matter in relativistic
hydrodynamics. Nuclear matter is described by a --type model for
the hadron matter phase and by the MIT bag model for the quark--gluon plasma,
with a first order phase transition between both phases. In the presence of
phase transitions, hydrodynamical solutions change qualitatively, for instance,
one-dimensional stationary compression is no longer accomplished by a single
shock but via a sequence of shock and compressional simple waves. We construct
the analytical solution to the ``slab-on-slab'' collision problem over a range
of incident velocities. The performance of numerical algorithms to solve
relativistic hydrodynamics is then investigated for this particular test case.
Consequences for the early compressional stage in heavy--ion collisions are
pointed out.Comment: 22 pages, 12 uuencoded figure
High-powered Gravitational News
We describe the computation of the Bondi news for gravitational radiation. We
have implemented a computer code for this problem. We discuss the theory behind
it as well as the results of validation tests. Our approach uses the
compactified null cone formalism, with the computational domain extending to
future null infinity and with a worldtube as inner boundary. We calculate the
appropriate full Einstein equations in computational eth form in (a) the
interior of the computational domain and (b) on the inner boundary. At future
null infinity, we transform the computed data into standard Bondi coordinates
and so are able to express the news in terms of its standard and
polarization components. The resulting code is stable and
second-order convergent. It runs successfully even in the highly nonlinear
case, and has been tested with the news as high as 400, which represents a
gravitational radiation power of about .Comment: 24 pages, 4 figures. To appear in Phys. Rev.
Thermal phenomenology of hadrons from 200 AGeV S+S collisions
We develop a complete and consistent description for the hadron spectra from
heavy ion collisions in terms of a few collective variables, in particular
temperature, longitudinal and transverse flow. To achieve a meaningful
comparison with presently available data, we also include the resonance decays
into our picture. To disentangle the influences of transverse flow and
resonance decays in the -spectra, we analyse in detail the shape of the
-spectra.Comment: 31 pages, 13 figs in seperate uuencoded file, for LaTeX, epsf.sty and
dvips, TPR-93-16 and BNL-(no number yet
Spin observables of the reactions NN -> DeltaN and pd -> Delta (pp)(1S0) in collinear kinematics
A general formalism for double and triple spin-correlations of the reaction
NN -> DeltaN is developed for the case of collinear kinematics. A complete
polarization experiment allowing to reconstruct all of the four amplitudes
describing this process is suggested. Furthermore, the spin observables of the
inelastic charge-exchange reaction pd -> Delta^0(pp)(1S0) are analyzed in
collinear kinematics within the single pN scattering mechanism involving the
subprocess pn -> Delta^0p. The full set of spin observables related to the
polarization of one or two initial particles and one final particle is obtained
in terms of three invariant amplitudes of the reaction pd -> Delta (pp)(1S0)
and the transition form factor d->(pp)(1S0). A complete polarization experiment
for the reaction pd -> Delta^0(pp)(1S0) is suggested which allows one to
determine three independent combinations of the four amplitudes of the
elementary subprocess NN -> DeltaN.Comment: 12 pages, 1 figur
Excitons in quasi-one dimensional organics: Strong correlation approximation
An exciton theory for quasi-one dimensional organic materials is developed in
the framework of the Su-Schrieffer-Heeger Hamiltonian augmented by short range
extended Hubbard interactions. Within a strong electron-electron correlation
approximation, the exciton properties are extensively studied. Using scattering
theory, we analytically obtain the exciton energy and wavefunction and derive a
criterion for the existence of a exciton. We also systematically
investigate the effect of impurities on the coherent motion of an exciton. The
coherence is measured by a suitably defined electron-hole correlation function.
It is shown that, for impurities with an on-site potential, a crossover
behavior will occur if the impurity strength is comparable to the bandwidth of
the exciton, corresponding to exciton localization. For a charged impurity with
a spatially extended potential, in addition to localization the exciton will
dissociate into an uncorrelated electron-hole pair when the impurity is
sufficiently strong to overcome the Coulomb interaction which binds the
electron-hole pair. Interchain coupling effects are also discussed by
considering two polymer chains coupled through nearest-neighbor interchain
hopping and interchain Coulomb interaction . Within the
matrix scattering formalism, for every center-of-mass momentum, we find two
poles determined only by , which correspond to the interchain
excitons. Finally, the exciton state is used to study the charge transfer from
a polymer chain to an adjacent dopant molecule.Comment: 24 pages, 23 eps figures, pdf file of the paper availabl
Improved Laboratory Transition Probabilities for Neutral Chromium and Re-determination of the Chromium Abundance for the Sun and Three Stars
Branching fraction measurements from Fourier transform spectra in conjunction
with published radiative lifetimes are used to determine transition
probabilities for 263 lines of neutral chromium. These laboratory values are
employed to derive a new photospheric abundance for the Sun: log (Cr
I) = 5.640.01 (). These Cr I solar abundances do
not exhibit any trends with line strength nor with excitation energy and there
were no obvious indications of departures from LTE. In addition, oscillator
strengths for singly-ionized chromium recently reported by the FERRUM Project
are used to determine: log (Cr II) = 5.770.03 (). Transition probability data are also applied to the spectra of three
stars: HD 75732 (metal-rich dwarf), HD 140283 (metal-poor subgiant), and CS
22892-052 (metal-poor giant). In all of the selected stars, Cr I is found to be
underabundant with respect to Cr II. The possible causes for this abundance
discrepancy and apparent ionization imbalance are discussed.Comment: 44 pages, 6 figure
Relativistic Hydrodynamics for Heavy--Ion Collisions -- I. General Aspects and Expansion into Vacuum
We present algorithms to solve relativistic hydrodynamics in 3+1--dimensional
situations without apparent symmetry to simplify the solution. In simulations
of heavy--ion collisions, these numerical schemes have to deal with the
physical vacuum and with equations of state with a first order phase transition
between hadron matter and a quark--gluon plasma. We investigate their
performance for the one--dimensional expansion of baryon-free nuclear matter
into the vacuum, which is an analytically solvable test problem that
incorporates both the aspect of the vacuum as well as that of a phase
transition in the equation of state. The dependence of the lifetime of the
mixed phase on the initial energy density is discussed.Comment: 31 pages, 16 uuencoded figure
Hydrodynamical assessment of 200 AGeV collisions
We are analyzing the hydrodynamics of 200 A GeV S+S collisions using a new
approach which tries to quantify the uncertainties arising from the specific
implementation of the hydrodynamical model. Based on a previous
phenomenological analysis we use the global hydrodynamics model to show that
the amount of initial flow, or initial energy density, cannot be determined
from the hadronic momentum spectra. We additionally find that almost always a
sizeable transverse flow deve- lops, which causes the system to freeze out,
thereby limiting the flow velocity in itself. This freeze-out dominance in turn
makes a distinction between a plasma and a hadron resonance gas equation of
state very difficult, whereas a pure pion gas can easily be ruled out from
present data. To complete the picture we also analyze particle multiplicity
data, which suggest that chemical equilibrium is not reached with respect to
the strange particles. However, the over- population of pions seems to be at
most moderate, with a pion chemical potential far away from the Bose
divergence.Comment: 19 pages, 11 figs in separate uuencoded file, for LateX, epsf.tex,
dvips, TPR-94-5 and BNL-(no number yet
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